The amount I'd need to pay for the whole build is 266.56€ since I have buch of parts ready.
I've tried looking around ebay and local online marketplaces for the old parts as cheap as possible.
My budget is 300€ and tbh I'd rather keep it even lower.

Anyway opinions about the build? Are the components good, should I change something? Any places where to find them even cheaper?

Splendid

The major problem you will have with this build is finding drivers for the devices. The driver architecture changed significantly between XP and Vista so newer drivers will not work with the XP devices and vice versa. You may have to purchase some old hardware just to get the drivers. BTW, do NOT buy any hardware that does not have XP compatible drivers included.

The major problem you will have with this build is finding drivers for the devices. The driver architecture changed significantly between XP and Vista so newer drivers will not work with the XP devices and vice versa. You may have to purchase some old hardware just to get the drivers. BTW, do NOT buy any hardware that does not have XP compatible drivers included.

For the pcie cards I've checked to make sure they are XP compatible and I think I checked that there's XP driver for GTX 680

Admirable

If your doing this just for fun ok,but any game for XP runs on newer windows and the few that don't have patches so they do run,and even if they don't have a patch you will be able to run them on XP mode under windows 7,you don't have to make a build for XP unless you really just want to do it for fun.
(And then have it stare at you from it's corner trying to make you feel guilty for not playing with it. )

If your doing this just for fun ok,but any game for XP runs on newer windows and the few that don't have patches so they do run,and even if they don't have a patch you will be able to run them on XP mode under windows 7,you don't have to make a build for XP unless you really just want to do it for fun.
(And then have it stare at you from it's corner trying to make you feel guilty for not playing with it. )

I know it's possible to play the games on current hardware but personally I like building PCs and having stuff like that physically be there. Idk it just has different feeling to it